Crystalline multinary-metal chalcogenide compositions, including particles and layers, are useful materials, having applications in catalysis and electronics, including uses as semiconductors, conductors, and thermoelectrics.
Crystalline multinary-metal chalcogenide compositions containing only non-toxic and abundant elements are of particular interest in developing environmentally sustainable processes and devices. Copper tin sulfide (Cu2SnS3 or “CTS”) and copper zinc tin sulfide (Cu2ZnSnS4 or “CZTS”) are particularly useful examples of this class of materials. CTS belongs to a group of compounds, represented by the general formula IB2-IVA-VIA3, which are of interest due to their potential applications as small band-gap semiconductors, as nonlinear materials, and as suitable candidates for photovoltaic cell materials.
Thin-film photovoltaic cells typically use semiconductors such as CdTe or copper indium gallium sulfide/selenide (CIGS) as an energy absorber material. Due to toxicity of cadmium and the limited availability of indium, alternatives are sought. CZTS possesses a band gap energy of about 1.5 eV and a large absorption coefficient (approx. 104 cm−1), making it a promising CIGS replacement.
Challenges in making CZTS thin-films are illustrative of the general challenges that must be surmounted in making films of crystalline multinary-metal chalcogenide compositions. Current techniques to make CZTS thin films (e.g., thermal evaporation, sputtering, hybrid sputtering, pulsed laser deposition and electron beam evaporation) require complicated equipment and therefore tend to be expensive. Electrochemical deposition is an inexpensive process, but compositional non-uniformity and/or the presence of secondary phases prevents this method from generating high-quality CZTS thin-films. CZTS thin-films can also be made by the spray pyrolysis of a solution containing metal salts, typically CuCl, ZnCl2, and SnCl4, using thiourea as the sulfur source. This method tends to yield films of poor morphology, density and grain size. Photochemical deposition has also been shown to generate p-type CZTS thin films. However, the composition of the product is not well-controlled, and it is difficult to avoid the formation of impurities such as hydroxides.
The synthesis of CZTS nanoparticles stabilized with organic amines and processed with organic solvents has also been disclosed. Layers of these nanoparticles were deposited on substrates by standard coating techniques. Subsequent annealing in a nitrogen and sulfur atmosphere leads to the formation of CZTS films. However, it is difficult to tune the molar ratio of elements in the CZTS powder, which affects the ultimate performance of the CZTS thin-film.
There still exists a need for a process that provides high-quality, crystalline, multinary-metal chalcogenide particles and films with tunable composition, size, and morphology. It is desirable to develop a low-cost, environmentally friendly process that is capable of producing these particles using water as the solvent. Processes for forming particles of compositions such as CTS and CZTS that are based upon abundant and non-toxic elements are of particular interest.